and Electrical Resistance of Iron. 215 



The heating was carried out electrically. For this purpose 

 the ring (which was made quite small, about an inch in 

 diameter) was covered with a non-inductively wound layer 

 ot insulated platinum wire. By passing a suitable current 

 through this wire the temperature could be raised to any 

 required extent. The heat was thus generated exactly where 

 wanted ; and by wrapping the ring thickly with asbestos 

 the loss of heat by radiation was made small, and the tem- 

 perature in the interior fairly uniform. 



In the experiments of M. Curie, 1500 watts were absorbed 

 in obtaining a temperature of 1350°: by the above method, 

 73 watts (a current of 4*6 amperes with 16 volts across the 

 terminals) was found sufficient to maintain the ring at 1150°; 

 and 45 watts sufficient at 800°, i. e., above the critical 

 temperature of the iron. Dealing thus with comparatively 

 small currents, the accurate regulation of the temperature 

 was rendered simple ; whilst absolute constancy for long 

 periods of the heating current, and therefore of the tempera- 

 ture, was, with a set of accumulators in good order, not 

 difficult to attain. 



To avoid oxidation of the iron, the ring-magnet was placed 

 in a glass vessel, and the electrical connexions brought 

 through a well-fitting cork rendered air-tight by a thick 

 layer of sealing-wax poured over it while hot. The oxygen 

 contained in this closed space could, previous to the experi- 

 ments, be absorbed by an auxiliary coil of bare iron wire, 

 heated to bright redness by an electric current. In the later 

 experiments the jar was, as a preliminary, exhausted by a 

 small air-pump to about a third of an atmosphere ; during 

 the heating, the pressure might rise above that of the atmo- 

 sphere, in which case the excess of heated air was allowed to 

 escape from a glass tube dipping into mercury; and this at 

 other times served as a gauge to show whether the glass 

 vessel was really air-tight. 



Details of the various Ring-Magnets. 



During the course of the experiments, four ring-magnets 

 were made. 



The first came quickly to grief through accidental over- 

 heating and partial oxidation of the iron. The oxide seems 

 to have combined with the silicates of the insulating materials, 

 forming a kind of slag, for the ring, on taking to pieces was 

 simply a collection of platinum wires buried in a brown 

 glassy substance with a little iron left in the core. 



The experiments with the second ring (whose core was 

 R2 



